This invention relates to a carrier regenerating or recovery device for use in a mobile station of a mobile satellite communication network.
In mobile satellite communication, a modulated signal is transmitted from a master station to mobile stations and from each mobile station to the master station. The modulated signal is derived by modulating a carrier signal at a frame period by a data signal representative of a message and by unique words which are periodically interspersed in the data signal. The modulated signal therefore comprises a modulated data signal and modulated unique words and will hereafter be referred to as a modulated overall signal. The unique words are preliminarily known at the mobile stations. Such a unique word will herein be called a locally known unique word.
In order to get primarily the message, each mobile station comprises a carrier regenerating device for regenerating the carrier signal as a regenerated carrier signal from the modulated overall signal received at the mobile station under consideration as an input modulated signal having the frame period and comprising the modulated data signal and the modulated unique words which are periodically interspersed in the modulated data signal. The carrier regenerating device comprises a demodulating circuit for using the regenerated carrier signal in quadrature demodulating the input modulated signal to produce a demodulated overall signal which comprises a demodulated data signal representative of the message and demodulated unique words. A conventional carrier regenerating or recovery circuit is for regenerating or producing the regenerated carrier signal from the demodulated overall signal. A carrier regenerating loop is therefore formed by the demodulating circuit and the carrier regenerating circuit.
The carrier regenerating loop is well operable insofar as the input modulated signal is supplied thereto steadily above a predetermined signal to noise (S/N) ratio. It should, however, be noted that the carrier regenerating loop may not always be supplied with the input modulated signal above the predetermined signal to noise ratio. For example, the mobile station moves from a location to another location and may be subjected to shadowing by buildings and tree depending on the location. In a worst case, the carrier regenerating loop can not receive the input modulated signal at all. In such an event, the carrier regenerating circuit is put into a free running state. The regenerated carrier signal steps out of frame synchronism, namely, out of synchronism with the frame period. As a result, a phase shift occurs in the regenerated carrier signal. In addition, a frequency shift may appear in the regenerated carrier signal.
The mobile station may move in due course to a location where the carrier regenerating loop can receive the input modulated signal above the predetermined signal to noise ratio. It takes, however, a long recovery time for the carrier regenerating circuit to regenerate the regenerated carrier signal again in synchronism with the frame period. This results in a long-continued and disagreeable click noise when the message is an audible one.